<?php
/**
 * CCache class file.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @link http://www.yiiframework.com/
 * @copyright Copyright &copy; 2008-2011 Yii Software LLC
 * @license http://www.yiiframework.com/license/
 */

/**
 * CCache is the base class for cache classes with different cache storage implementation.
 *
 * A data item can be stored in cache by calling {@link set} and be retrieved back
 * later by {@link get}. In both operations, a key identifying the data item is required.
 * An expiration time and/or a dependency can also be specified when calling {@link set}.
 * If the data item expires or the dependency changes, calling {@link get} will not
 * return back the data item.
 *
 * Note, by definition, cache does not ensure the existence of a value
 * even if it does not expire. Cache is not meant to be a persistent storage.
 *
 * CCache implements the interface {@link ICache} with the following methods:
 * <ul>
 * <li>{@link get} : retrieve the value with a key (if any) from cache</li>
 * <li>{@link set} : store the value with a key into cache</li>
 * <li>{@link add} : store the value only if cache does not have this key</li>
 * <li>{@link delete} : delete the value with the specified key from cache</li>
 * <li>{@link flush} : delete all values from cache</li>
 * </ul>
 *
 * Child classes must implement the following methods:
 * <ul>
 * <li>{@link getValue}</li>
 * <li>{@link setValue}</li>
 * <li>{@link addValue}</li>
 * <li>{@link deleteValue}</li>
 * <li>{@link flush} (optional)</li>
 * </ul>
 *
 * CCache also implements ArrayAccess so that it can be used like an array.
 *
 * @author Qiang Xue <qiang.xue@gmail.com>
 * @version $Id: CCache.php 3001 2011-02-24 16:42:44Z alexander.makarow $
 * @package system.caching
 * @since 1.0
 */
abstract class CCache extends CApplicationComponent implements ICache, ArrayAccess
{
    /**
     * @var string a string prefixed to every cache key so that it is unique. Defaults to {@link CApplication::getId() application ID}.
     */
    public $keyPrefix;

    /**
     * Initializes the application component.
     * This method overrides the parent implementation by setting default cache key prefix.
     */
    public function init()
    {
        parent::init();
        if($this->keyPrefix===null)
        $this->keyPrefix=Yii::app()->getId();
    }

    /**
     * @param string $key a key identifying a value to be cached
     * @return sring a key generated from the provided key which ensures the uniqueness across applications
     */
    protected function generateUniqueKey($key)
    {
        return md5($this->keyPrefix.$key);
    }

    /**
     * Retrieves a value from cache with a specified key.
     * @param string $id a key identifying the cached value
     * @return mixed the value stored in cache, false if the value is not in the cache, expired or the dependency has changed.
     */
    public function get($id)
    {
        if(($value=$this->getValue($this->generateUniqueKey($id)))!==false)
        {
            $data=unserialize($value);
            if(!is_array($data))
            return false;
            if(!($data[1] instanceof ICacheDependency) || !$data[1]->getHasChanged())
            {
                Yii::trace('Serving "'.$id.'" from cache','system.caching.'.get_class($this));
                return $data[0];
            }
        }
        return false;
    }

    /**
     * Retrieves multiple values from cache with the specified keys.
     * Some caches (such as memcache, apc) allow retrieving multiple cached values at one time,
     * which may improve the performance since it reduces the communication cost.
     * In case a cache doesn't support this feature natively, it will be simulated by this method.
     * @param array $ids list of keys identifying the cached values
     * @return array list of cached values corresponding to the specified keys. The array
     * is returned in terms of (key,value) pairs.
     * If a value is not cached or expired, the corresponding array value will be false.
     * @since 1.0.8
     */
    public function mget($ids)
    {
        $uniqueIDs=array();
        $results=array();
        foreach($ids as $id)
        {
            $uniqueIDs[$id]=$this->generateUniqueKey($id);
            $results[$id]=false;
        }
        $values=$this->getValues($uniqueIDs);
        foreach($uniqueIDs as $id=>$uniqueID)
        {
            if(!isset($values[$uniqueID]))
            continue;
            $data=unserialize($values[$uniqueID]);
            if(is_array($data) && (!($data[1] instanceof ICacheDependency) || !$data[1]->getHasChanged()))
            {
                Yii::trace('Serving "'.$id.'" from cache','system.caching.'.get_class($this));
                $results[$id]=$data[0];
            }
        }
        return $results;
    }

    /**
     * Stores a value identified by a key into cache.
     * If the cache already contains such a key, the existing value and
     * expiration time will be replaced with the new ones.
     *
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @param ICacheDependency $dependency dependency of the cached item. If the dependency changes, the item is labeled invalid.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     */
    public function set($id,$value,$expire=0,$dependency=null)
    {
        Yii::trace('Saving "'.$id.'" to cache','system.caching.'.get_class($this));
        if($dependency!==null)
        $dependency->evaluateDependency();
        $data=array($value,$dependency);
        return $this->setValue($this->generateUniqueKey($id),serialize($data),$expire);
    }

    /**
     * Stores a value identified by a key into cache if the cache does not contain this key.
     * Nothing will be done if the cache already contains the key.
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @param ICacheDependency $dependency dependency of the cached item. If the dependency changes, the item is labeled invalid.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     */
    public function add($id,$value,$expire=0,$dependency=null)
    {
        Yii::trace('Adding "'.$id.'" to cache','system.caching.'.get_class($this));
        if($dependency!==null)
        $dependency->evaluateDependency();
        $data=array($value,$dependency);
        return $this->addValue($this->generateUniqueKey($id),serialize($data),$expire);
    }

    /**
     * Deletes a value with the specified key from cache
     * @param string $id the key of the value to be deleted
     * @return boolean if no error happens during deletion
     */
    public function delete($id)
    {
        Yii::trace('Deleting "'.$id.'" from cache','system.caching.'.get_class($this));
        return $this->deleteValue($this->generateUniqueKey($id));
    }

    /**
     * Deletes all values from cache.
     * Be careful of performing this operation if the cache is shared by multiple applications.
     * @return boolean whether the flush operation was successful.
     */
    public function flush()
    {
        Yii::trace('Flushing cache','system.caching.'.get_class($this));
        return $this->flushValues();
    }

    /**
     * Retrieves a value from cache with a specified key.
     * This method should be implemented by child classes to retrieve the data
     * from specific cache storage. The uniqueness and dependency are handled
     * in {@link get()} already. So only the implementation of data retrieval
     * is needed.
     * @param string $key a unique key identifying the cached value
     * @return string the value stored in cache, false if the value is not in the cache or expired.
     * @throws CException if this method is not overridden by child classes
     */
    protected function getValue($key)
    {
        throw new CException(Yii::t('yii','{className} does not support get() functionality.',
        array('{className}'=>get_class($this))));
    }

    /**
     * Retrieves multiple values from cache with the specified keys.
     * The default implementation simply calls {@link getValue} multiple
     * times to retrieve the cached values one by one.
     * If the underlying cache storage supports multiget, this method should
     * be overridden to exploit that feature.
     * @param array $keys a list of keys identifying the cached values
     * @return array a list of cached values indexed by the keys
     * @since 1.0.8
     */
    protected function getValues($keys)
    {
        $results=array();
        foreach($keys as $key)
        $results[$key]=$this->getValue($key);
        return $results;
    }

    /**
     * Stores a value identified by a key in cache.
     * This method should be implemented by child classes to store the data
     * in specific cache storage. The uniqueness and dependency are handled
     * in {@link set()} already. So only the implementation of data storage
     * is needed.
     *
     * @param string $key the key identifying the value to be cached
     * @param string $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     * @throws CException if this method is not overridden by child classes
     */
    protected function setValue($key,$value,$expire)
    {
        throw new CException(Yii::t('yii','{className} does not support set() functionality.',
        array('{className}'=>get_class($this))));
    }

    /**
     * Stores a value identified by a key into cache if the cache does not contain this key.
     * This method should be implemented by child classes to store the data
     * in specific cache storage. The uniqueness and dependency are handled
     * in {@link add()} already. So only the implementation of data storage
     * is needed.
     *
     * @param string $key the key identifying the value to be cached
     * @param string $value the value to be cached
     * @param integer $expire the number of seconds in which the cached value will expire. 0 means never expire.
     * @return boolean true if the value is successfully stored into cache, false otherwise
     * @throws CException if this method is not overridden by child classes
     */
    protected function addValue($key,$value,$expire)
    {
        throw new CException(Yii::t('yii','{className} does not support add() functionality.',
        array('{className}'=>get_class($this))));
    }

    /**
     * Deletes a value with the specified key from cache
     * This method should be implemented by child classes to delete the data from actual cache storage.
     * @param string $key the key of the value to be deleted
     * @return boolean if no error happens during deletion
     * @throws CException if this method is not overridden by child classes
     */
    protected function deleteValue($key)
    {
        throw new CException(Yii::t('yii','{className} does not support delete() functionality.',
        array('{className}'=>get_class($this))));
    }

    /**
     * Deletes all values from cache.
     * Child classes may implement this method to realize the flush operation.
     * @return boolean whether the flush operation was successful.
     * @throws CException if this method is not overridden by child classes
     * @since 1.1.5
     */
    protected function flushValues()
    {
        throw new CException(Yii::t('yii','{className} does not support flushValues() functionality.',
        array('{className}'=>get_class($this))));
    }

    /**
     * Returns whether there is a cache entry with a specified key.
     * This method is required by the interface ArrayAccess.
     * @param string $id a key identifying the cached value
     * @return boolean
     */
    public function offsetExists($id)
    {
        return $this->get($id)!==false;
    }

    /**
     * Retrieves the value from cache with a specified key.
     * This method is required by the interface ArrayAccess.
     * @param string $id a key identifying the cached value
     * @return mixed the value stored in cache, false if the value is not in the cache or expired.
     */
    public function offsetGet($id)
    {
        return $this->get($id);
    }

    /**
     * Stores the value identified by a key into cache.
     * If the cache already contains such a key, the existing value will be
     * replaced with the new ones. To add expiration and dependencies, use the set() method.
     * This method is required by the interface ArrayAccess.
     * @param string $id the key identifying the value to be cached
     * @param mixed $value the value to be cached
     */
    public function offsetSet($id, $value)
    {
        $this->set($id, $value);
    }

    /**
     * Deletes the value with the specified key from cache
     * This method is required by the interface ArrayAccess.
     * @param string $id the key of the value to be deleted
     * @return boolean if no error happens during deletion
     */
    public function offsetUnset($id)
    {
        $this->delete($id);
    }
}